PM / devfreq: GPU load with reference to max frequency

Calculate relative GPU load with respect to maximum
GPU frequency and average time for which GPU is in
suspend state in a sampling interval controlled by
the client. Average load and suspend time are
exposed via sysfs.

Change-Id: I3510cf5d4611e2e80e75f6bf13043734e50d37e3
Signed-off-by: Sunil Khatri <sunilkh@codeaurora.org>